Molecular Characterization of Contaminants in Soils by Spatially Resolved XRF & XANES Spectroscopy

Abstract
Using a dedicated microprobe beam line (X-26A NSLS) we have conducted spatially resolved XRF and XAS studies on a variety of environmental samples and on samples produced in waste isolation and processing activities. XANES spectroscopy has provided detailed information on the oxidation states of U in contaminated soils on regions as small as 50 µm, with minimal sample manipulation. Qualitative information on bonding environments of U have been deduced based upon elemental associations generated at 10 µm spatial resolution on identical regions where complete XANES spectra were collected and also on the 'fingerprint' or multiple scattering regions of the XANES spectra. Distinct regions within the sediments were found to contain phases of predominately U(VI) and U(IV). On the spatial scale examined, the U(VI) was present both as diffusely distributed and as discrete phases, whereas phases containing predominately U(IV) were present exclusively as discrete phases. Examination of a large population of individual grains prior to and following extraction with a sodium carbonate lixiviant, revealed that the U(IV) phases were highly enriched following extraction, suggesting more selective solubilization of U(VI) phases. This study illustrates the utility of combining spatially resolved XRF and XANES to evaluate chemical intervention technologies.